This invention relates generally to micron-scale epidermal abrasion devices. More particularly, this invention relates to a micron-scale epidermal abrasion device mold that may be used to form low-cost epidermal abrasion devices.
The biomedical industry seeks to replace stainless steel hypodermic injection needles with needles that have small diameters, sharper tips, and which can provide additional functionality. The advantages of smaller diameters and sharper tips are to minimize pain and tissue damage. Desirable additional functionality for a hypodermic injection needle includes the capability of providing integrated electronics for chemical concentration monitoring, cell stimulation, and the control of fluid flow, such as through an integrated valve or pump.
Integrated circuit technology and single crystal silicon wafers have been used to produce hypodermic injection needles. A xe2x80x9cmicrohypodermicxe2x80x9d injection needle or xe2x80x9cmicroneedlexe2x80x9d is described in Lin, et al., xe2x80x9cSilicon Processed Microneedlexe2x80x9d, Digest of Transducers ""93, International Conference on Solid-State Sensors and Actuators, pp. 237-240, June 1993. Another microneedle is described in Chen and Wise. xe2x80x9cA Multichannel Neural Probe for Selective Chemical Delivery at the Cellular Level,xe2x80x9d Technical Digest of the Solid-State Sensor and Actuator Workshop, Hilton head Island, S.C., pp. 256-259, Jun. 13-16, 1994. The needles described in these references have common elements since they are both based on the process flow for a multielectrode probe. In particular, both processes rely on heavily boron doped regions to define the shape of the needle and the utilization of ethylenediamine pyrocatechol as an anisotropic etchant.
Lin, et al. describe a fluid passage that is surface micromachined and utilizes a timed etch to thin the wafer such that an approximately 50 xcexcm thick strengthening rib of single crystal silicon remains. In contrast, Chen and Wise bulk micromachine a channel into the microneedle using an anisotropic etch and all of the single crystal silicon comprising the shaft of the needle is heavily boron doped so the timing of the anisotropic etch to form the shape of the needle is less critical.
There are a number of disadvantages associated with these prior art devices. The single crystal silicon strengthening rib in the Lin, et al. microneedle is naturally rough and is difficult to reproduce due to the tight tolerance on the timed etch. The Chen and Wise microneedle results in walls approximately 10 xcexcm or less in thickness and the shape of the fluid channel defines the shape of the silicon comprising the structural portion of the needle. Therefore, small channels lead to thin needles and large channels lead to large needles. This is a problem when a needle with a small channel but large needle cross-section is desired. Often, large needle cross-sections are necessary, such as those 50 xcexcm thick or greater, to obtain a stronger microneedle, but since the fluid flow rate is dependent on the cross-section of the needle, a large needle may not provide the necessary flow resistance. To establish the necessary flow resistance in a large needle cross-section, a complicated nested channel configuration must be fabricated.
The Lin, et al. and Chen and Wise microneedles share the drawback that they rely on the use of boron doping to define the shape of the needle. This requires a long (approximately 8 hours in Chen and Wise; approximately 16 hours in Lin), high temperature (approximately 1150xc2x0 C.) step which is expensive. In addition, the chosen anisotropic etchant is ethylenediamine pyrocatechol, which is a strong carcinogen, making production dangerous and therefore leading to further expenses. Finally, since both of these microneedles utilize an anisotropic etchant to produce the shape of the microneedle, limitations are placed on the geometry of the needle. For the needle to be xe2x80x9csharpestxe2x80x9d, it is preferred for the tip of the needle to originate from a near infinitesimally small point and taper continuously, without step transitions, to the full width of the shaft of the needle. Such a geometry is not possible using the techniques described in Lin, et al. and Chen and Wise. In particular, the needles produced using those techniques have abrupt step transitions, largely attributable to the use of the anisotropic etchant.
Microneedles that do not include a channel are referred to herein as lancets. Lancets may be used to lance the epidermis so that a drop of blood can be sampled. Lancets may also be formed in configurations that allow them be used as blades or scalpels. Such devices can be used for cutting skin, eyes or other tissue in a surgical context. Thus, as used herein, a transdermal probe refers to microneedles, lancets, or blades (scalpels).
An array of microneedles may be formed on a silicon substrate. The array of microneedles can be subsequently used as an xe2x80x9cabraderxe2x80x9d. That is, the device may be used to abrade the epidermis to facilitate transdermal drug delivery. The problem with forming an abrader from silicon is that it is relatively expensive. Therefore, it would be highly desirable to identify a low cost technique for fabricating an abrader. Ideally, such a technique would rely upon known manufacturing processes and equipment.
A method of forming an injection molded epidermal abrasion device includes depositing mold material on an epidermal abrasion device. The epidermal abrasion device is separated from the mold material to yield a mold. An epidermal abrasion device is then formed within the mold. The epidermal abrasion device may include a matrix of isotropically etched structures having isotropically etched sidewalls positioned between wide bases and narrow tips, each isotropically etched structure having a vertical height of at least 20 xcexcm. The matrix of isotropically etched structures may define a matrix of pyramids.